Alkalinity pyrite deposition

STANNITE (Mineral). This mineral is a sulfo-stannate of copper and non, sometimes with some zinc, corresponding to the formula. Uii PeSnS j It is tetragonal brittle with uneven fracture hardness, 4 specific gravity, 4.3-4.5 metallic luster color, gray to black, sometimes tarnished by chalcopyiite stieak, black opaque. The mineral occuts associated witli cassilerile, chalcopyiite. tetrahedrite, and pyrite, probably the result of deposition by hot alkaline solution. Stannite occurs in Bohemia Cornwall, England Tasmania Bolivia, and in the United States in South Dakota. It derives its name from the Latin word for tin, stannum. 

At the ordinary temperature this reaction is very slow, but at 200° C. it is fairly rapid. The dark deposit is microcrystalline. On carrying out the reaction in a sealed tube at 100° C. with a solution containing 1 per cent, of free sulphuric acid, marcasite is the only product. Higher temperatures and reduction of acidity favour the production of pyrites, distinct crystals being produced at 200° C. Iron pyrites is formed in neutral or alkaline solutions, as, for example, by the action of sodium polysulphide on a ferious salt. Marcasite is not produced under these conditions.1 The foregoing results are in harmony with the observation that whilst iron pyrites in nature is usually formed in deep veins from hot alkaline solutions, marcasite is produced near the surface from acid solutions.2... 

Thus a recent formation of pyrites has been observed at Karlsbad, in the well-known springs, which have a temperature of about 55° C. The waters are faintly alkaline and contain dissolved sulphates and a trace of hydrogen sulphide.3 Similarly the Tuscan lagoons are gradually depositing pyrites, whilst the hot vapours of the Icelandic fumaroles are slowly converting the ferrous silicate of the rocks into pyrites.4... 

The relationships between the silicate and sulfide sediments are also determined mainly by the content of sulfur in the water, and pH and Eh. For given sulfur activities the sulfide field is curtailed due to formation of silicates only in highly alkaline environments (pH >11). Deposition of silicates in environments close to neutral occurs in the stabihty field of magnetite. Magnetite is not formed in primary sediments in the presence of active forms of Si02. As the sulfur content in the waters decreases, the boundaries between the iron sulfide and iron sihcate fields shift toward neutral environments. In waters with pH = 8 pyrrhotite begins to be replaced by silicates at < 10and pyrite at Og 10 g-ion/1. 

Besshi deposits comprise thin sheets of massive, well-layered pyrrhotite - - chalcopyrite + sphalerite - - pyrite with minor galena and cobalt minerals interlayered with terrigenous clastic sedimentary rocks and calc-alkaline... 

Primary minerals formed in the ore deposit prior to weathering and erosion, including a wide variety of metal sulfides and sulfosalts, metal oxides, metal- and alkaline-earth carbonates, sulfates, crystalline silica, clays, and other silicates. Many metal sulfides (especially iron sulfides such as pyrite), when exposed by erosion or mining to atmospheric oxygen and water, can form acid-rock drainage (ARD). 

Low-grade uranium deposits occur in the Quaternary alkaline volcanics of northern Latium (Fig. 10). Thin stratiform peneconcordant uranium mineralization is found in reworked loose tuffaceous deposits and, locally, in diatomite layers. Widespread kaolinization and pyrite-marcasite mineralization occur in the host volcanic beds." Uranium mineralization is related to the supergene environment. 

Lapis lazuli, or simply lapis, is a rock composed of a mixture of minerals, usually calcite, pyrite and lazurite (qq.v.), with the latter being the dominant constituent. Occurrence is as a blue rock containing brassy yellow crystals (pyrite) with white streaks due to calcite. It is formed from the metamorphism of limestones by an alkaline igneous intrusion (Deer et al., 1992 Rutley, 1988). Other silicate minerals such as hatiyne, sodalite, diopside, forsterite, muscovite and wollastonite (. v.) may also be present. The term is often used incorrectly to refer to the mineral lazurite, which is only one of its components. Lapis lazuli is most famously known from the deposit in the Kokcha River valley, Afghanistan, where it has been mined for more than 6000 years for use in jewellery, as an ornamental stone and as a pigment, ft is also known from Mt Vesuvius (Italy), Colorado and California (USA), Siberia and Chile. 

---